diff --git a/libavcodec/assdec.c b/libavcodec/assdec.c
index 3178f2953c..f0b1069cd2 100644
--- a/libavcodec/assdec.c
+++ b/libavcodec/assdec.c
@@ -31,7 +31,8 @@ static av_cold int ass_decode_init(AVCodecContext *avctx)
     avctx->subtitle_header = av_malloc(avctx->extradata_size + 1);
     if (!avctx->subtitle_header)
         return AVERROR(ENOMEM);
-    memcpy(avctx->subtitle_header, avctx->extradata, avctx->extradata_size);
+    if (avctx->extradata_size)
+        memcpy(avctx->subtitle_header, avctx->extradata, avctx->extradata_size);
     avctx->subtitle_header[avctx->extradata_size] = 0;
     avctx->subtitle_header_size = avctx->extradata_size;
     return 0;